The present invention relates to a process for preparing a rare earth borate by reacting together at least one rare earth salt, boric acid and a base.
Major developments are currently being made in the luminescence and electronics fields. Examples of such developments are the perfection of plasma systems (screens and lamps) for new viewing and lighting techniques. One particular application is that of replacing current television screens with flat screens. Such novel applications require luminophore materials with constantly improving properties. Rare earth borates constitute materials of that type.
The borates must have good phase purity and good crystallinity to be able to be used in the applications mentioned above.
Processes for preparing such borates are known which involve solid state reaction of rare earth oxides and boric acid. Such processes have the disadvantage of leading to products which are not phase pure or which cannot be calcined at high temperatures. This latter point constitutes a problem as the luminous yield of luminophores depends on this temperature and is higher at higher temperatures.
One aim of the present invention is to develop a process which does not have these disadvantages.
To this end, the invention provides a process for preparing a rare earth borate by reacting together at least one rare earth salt, boric acid and a base, characterized in that the reaction is carried out using an excess of acid with respect to the stoichiometry and under conditions such that the pH of the reaction medium at the end of the reaction is in the range 6 to 8, recovering the precipitate formed then calcining it.
Other characteristics, details and advantages of the invention will become apparent from the following description and the non limiting examples which illustrate it.
The process of the invention concerns the preparation of rare earth borates. The term xe2x80x9crare earthxe2x80x9d as used in the present description means elements from the group constituted by yttrium, scandium and elements from the periodic table with an atomic number in the range 57 to 71 inclusive.
The term xe2x80x9crare earth boratexe2x80x9d means products with formula LnBO3 (orthoborate). Ln represents the rare earth as defined above. It should be understood that the process of the present invention can also be used to prepare mixed rare earth borates with formula LnBO3, Ln being constituted by a mixture of two or more rare earths. The process of the present invention can in particular be used to prepare a borate of yttrium, lanthanum or gadolinium.
The process can also be used to prepare doped rare earth borates with formula Ln1xe2x88x92xMxBO3, where M represents a doping element. This doping element is intended to endow the borate with luminescence properties or to reinforce them and it can be selected from the group formed by antimony, bismuth, cerium, terbium, lanthanum, gadolinium, europium, thulium, erbium and praseodymium. Since rare earths appear in this group, it goes without saying that the rare earth used as the doping element will be different from the rare earth constituting the borate.
The process of the invention consists of reacting a rare earth salt, boric acid and a base. The rare earth salt can be an inorganic or organic salt. Water-soluble salts are preferably used. More particularly, the rare earth salt is the nitrate. When preparing mixed borates, of course, a salt of each of the rare earths concerned is used.
In particular, the base is ammonia.
When a doped borate is prepared, the reaction is carried out in the presence of a salt of that element. The foregoing with respect to rare earth salts is applicable in this instance. Normally, the conditions used are such that the amount of doping element is at most 50 mole % with respect to the rare earth borate, more particularly at most 20%.
The reaction is preferably carried out with heat, for example at a temperature in the range 40xc2x0 C. to 90xc2x0 C.
One characteristic of the process of the invention is the use of an excess of boric acid for the reaction between the rare earth salt and boric acid. The term xe2x80x9cexcessxe2x80x9d means that the B/Ln atomic ratio is greater than 1. Generally, the ratio is limited to at most 50% (i.e., a B/Ln ratio of 1.5), more particularly to at most 30%. Higher ratios have the disadvantage of requiring the elimination of a large quantity of boric acid at the end of the reaction, which renders the process of less interest from an industrial viewpoint. Normally, an excess of 10% to 30% boric acid, more particularly 20% or about 20%, is used.
A further characteristic of the process of the invention is that the pH of the reaction medium at the end of the reaction is in the range 6 to 8. This pH value can depend on the nature of the rare earth in the borate which is to be prepared. Thus when the rare earth is lanthanum, the pH is preferably in the range 7.5 to 8. For gadolinium, the pH is preferably in the range 6.5 to 7. Finally, for yttrium, the pH is preferably 7 or about 7. The pH values given above are those which result in products with good phase purity as easily as possible.
In one particular implementation of the invention, the reaction is carried out by forming a mixture comprising at least one rare earth salt and boric acid. After any necessary homogenisation of the mixture formed, the base is added to the mixture to produce the desired pH. A precipitate then forms.
Once precipitation is complete, the reaction medium can be aged if necessary.
The precipitate is separated from the reaction medium using any suitable means.
The precipitate obtained can be washed and optionally dried, for example oven dried. The precipitate can be ground if necessary.
The precipitate is then calcined. Calcining is carried out at a temperature which is sufficient to obtain the borate in a crystalline form.
The precipitate can be calcined at a temperature in the range 1000xc2x0 C. to 1500xc2x0 C., to produce a borate with a good luminescence yield.
It may be of interest to carry out a first calcining step at a lower temperature, in particular of the order of 500xc2x0 C., to eliminate impurities, for example ammonium nitrate type impurities, then to carry out a second calcining step at a temperature in the range given above.
A non limiting example will now be given.